Our invention relates generally to an electric protective circuit for triggering a relatively high current, high voltage, solid state controlled switching device when a forward bias voltage of predetermined excess magnitude is impressed on the device, and more particularly it relates to improvements in the thyristor overvoltage protective circuit described and claimed in U.S. Pat. No. 3,662,250 granted to Piccone and Somos on May 9, 1972 and assigned to General Electric Company.
"Thyristor" is a generic name for a family of solid state bistable switches, including silicon controlled rectifiers (SCRs), which are physically characterized by a semiconductor wafer having a plurality of layers of alternately P and N type conductivities disposed between a pair of main current carrying metallic electrodes (designated the anode and cathode) and provdided with a control or "gating " electrode. When connected in series with a load impedance and subjected to a forward bias voltage (anode potential positive with respect to the cathode), a thyristor will ordinarily block the flow of load current until triggered or "fired" by the application to its gating electrode of an appropriate control signal, whereupon it abruptly switches from a high resistance blocking state to a very low resistance forward conducting or "on" state. Subsequently the device reverts to its non-conducting (turned off) state in response to through current being reduced below a given holding level.
The forward current and peak blocking voltage ratings of a thyristor are specified by the manufacturer. These ratings determine, under stated conditions and without damaging the thyristor, the maximum load current that the thyristor can conduct when "on" and the maximum applied voltage that it can safely withstand when "off." High current ratings are generally obtained by using relatively large area semiconductor wafers, while high voltage ratings require relatively thick base layers in the wafers. Thus, by way of example, a thyristor having a forward current rating of 1250 amperes average and a repetitive peak forward blocking voltage rating of 2600 volts at an operating junction temperature of 70.degree. C may have a wafer whose area is approximately 3.0 square inches and whose thickness is approximately 0.03 inch. For higher voltage applications, a plurality of such thyristors can be interconnected in series and operated in unison to form a solid state controllable electric valve. One such application is in the field of high voltage direct current power transmission where a plurality of such valves are interconnected and arranged to form a high current converter for controlling the flow of fault electric power between DC and AC sections of a high voltage power transmission system.
During those cyclically recurring intervals when the above mentioned converter valve is in an "off" or blocking state, the valve and its associated equipment are prone to being damaged by extra high voltage surges that may be produced by a variety of different transient phenomena, such, for example, as lightning strokes, bushing flashovers or inverter commutation failures. Lightning arrestors are commonly used to harmlessly divert and suppress overvoltage transients, but it is believed impractical and unwise to rely solely on such arrestors to protect solid state valves when exposed to abnormal voltage surges in the forward direction. In addition, since the arrestor is usually connected across an entire valve consisting of several semiconductor devices in series, there is no guarantee that each constituent thyristor of the valve will not be individually subject to excessive voltage. If a surge of forward anode voltage on an individual thyristor were to increase to a critical level above rated peak off-state voltage, the thyristor will turn on due to a voltage breakover. This mode of turn on, which can be caused by an avalanche breakdown, a punch through or excessive leakage, is a known phenomenum in the thyristor art. It is also known that the normal di/dt capabilities of conventional high voltage thyristors (e.g., thyristors having peak blocking voltages over 1500 volts) are greatly reduced when turned on in this mode.
In our foregoing prior patent, an improved overvoltage responsive trigger scheme is disclosed for protecting a high power main thyristor from forward voltage breakover. The protective circuit comprises a plurality of lower voltage auxiliary PN-PN semiconductor elements connected between the anode and gate of the main thyristor and a series L-C circuit connected between the gate and the cathode. The auxiliary PN-PN elements are selected to turn on in a voltage breakover mode when the forward bias voltage on the main thyristor attains a predetermined threshold magnitude which is lower than the breakover level of the main thyristor, whereupon the latter is triggered by a sharp gate pulse before the voltage attains a destructively high level.
It is found in practice that the overvoltage protective circuit disclosed in my foregoing patent is limited to those discrete, aggregate voltage breakover levels which can be attained by selected combinations of commercially available auxiliary thyristors. Such auxiliary thyristors are available with voltage breakover ratings spaced-apart in magnitude, so that available series combinations of such thyristors are also characterized by aggregate voltage breakover values spaced apart in steps. Intermediate voltage breakover levels may thus be unattainable in practice, or may be attained only by rearranging the series combination of auxiliary thyristors to include one or more high voltage devices in place of several less expensive low voltage devices. It is desirable to provide means for reducing or increasing the effective series breakover level of a series of auxiliary thyristors in steps smaller than attainable my omitting or adding one thyristor in the series.